This invention is concerned generally with reflecting telesopes, and more particularly with a correcting lens for such telescopes.
A common telescope widely used by amateur astonomers and professional astronomers is the Newtonian reflecting telescope. In such telescopes a parabolic mirror is positioned at one end of a supporting tube to collect incoming light and focus that light to an image point which can be examined by the viewer. Typically, the light is reflected by a small, flat mirror out to a region at the side of the tube, so that the image may be viewed through an eyepiece mounted at the side of the telescope tube. In recent years Newtonian telescopes of this type have become available with much larger apertures than heretofore. For example, telescopes with an aperture speed of f/2.9 are now commercially available. These fast Newtonian telescopes present aberration problems that were non-existent or less important with optically slower telescopes of this kind.
One aberration is "coma", an aberration which affects light rays off the optical axis of the telescope; in particular, stars which are off-axis take on the appearance of comets, having a tail extending outwardly from a central bright spot. A detailed discussion of the coma aberration for a spherical lens can be found in the textbook "Fundamentals of Optics" by Jenkins and White, published in 1957 by McGraw-Hill Book Company. An excellent discussion of the coma aberration of parabolic mirrors may be found in "Telescope Making" (ISS N 0190-5570), No. 9, Richard Berry, Editor; published in the Fall Quarter of 1980 by Astromedia Corporation. A discussion may also be found in the "Handbook of Military Infrared Technology," William L. Wolfe, Editor; published in 1965 by the Office of Naval Research Department of the Navy, Washington, D.C.
In addition to the problem of coma of the parabolic mirror, it has been found that the use of any eyepiece with the fast Newtonian telescopes generates additional aberrations by virtue of the eyepiece itself. These aberrations are both spherical aberration and coma. The resulting image quality is therefore greatly degraded.
A great body of literature exists concerning the correction of aberrations in lens and mirror systems of all kinds. In general, correction of aberrations involves the placing of a number of correcting lenses in the optical system, the various correcting lenses being designed to more or less cancel out the most objectionable aberrations of the other lenses in the system, including new aberrations introduced by the correcting lenses themselves.
A reference of particular interest with respect to the present invention is the paper entitled "New Catadioptric Meniscus Systems" by D. D. Maksutov, published in the Journal of the Optical Society of America, Volume 34, No. 5, May 1944. The Maksutov paper is largely concerned with the use of meniscus lenses to correct aberrations of spherical reflecting mirrors used in telescopes. The paper does not deal with correcting aberrations of parabolic reflectors, nor with correcting aberrations of both reflectors and viewing lenses (eyepieces) in a telescope.
Other references showing coma correction are: "Field Correctors for Large Telescopes," Applied Optics, September 1965, by C. G. Wynne; "Corrected Cassegraim System," Applied Optics, January 1964, by Seymore Rosin; and "Ritchey Chretian Corrector System," Applied Optics, April 1966 by Seymore Rosin.